Some commercial and industrial activities require systems that enable the storage and retrieval of a large number of different products. One known system for the storage and retrieval of items in multiple product lines involves arranging storage bins or containers on rows of shelves arranged in aisles. Each bin or container holds a plurality of products of one product type. The aisles provide access between the rows of shelves, so that the required products can be retrieved by operatives or robots that circulate in the aisles. It will be appreciated, however, that the need to provide aisle space to access the products means that the storage density of such systems is relatively low. In other words, the amount of space actually used for the storage of products is relatively small compared to the amount of space required for the storage system as a whole.
In an alternative approach, which offers a significant improvement in storage density, containers are stacked on top of one another and the stacks are arranged in rows. The containers are accessed from above, removing the need for aisles between the rows and allowing more containers to be stored in a given space.
Methods of handling containers stacked in rows have been well known for decades. In some such systems, for example as described in U.S. Pat. No. 2,701,065, free-standing stacks of containers are arranged in rows in order to reduce the storage volume associated with storing such containers while still providing access to a specific container if required. Access to a given container is made possible by providing relatively complicated hoisting mechanisms which can be used to stack containers and to remove given containers from stacks. The cost of such systems are, however, impractical in many situations and they have mainly been commercialised for the storage and handling of large shipping containers.
The concept of using freestanding stacks of containers and providing a mechanism to retrieve and store specific containers has been developed further, for example as described in EP 0 767 113 B (Cimcorp). Cimcorp discloses a mechanism for removing a plurality of stacked containers using a robotic load handler in the form of a rectangular tube which is lowered around the stack of containers, and which is configured to be able to grip a container at any level in the stack. In this way, several containers can be lifted at once from a stack. The movable tube can be used to move several containers from the top of one stack to the top of another stack, or to move containers from a stack to an external location and vice versa. Such systems can be particularly useful where all of the containers in a single stack contain the same product (known as a single-product stack). The load handler can be used to move containers between single-product stacks, for example to add a plurality of containers containing a single type of product to the store, and to pick up one or more containers from two or more single-product stacks to create a multi-product output stack. An example of this is the picking of vegetable crates in a central warehouse to create a multi-product order for delivery to retail stores.
In the system described in Cimcorp, the height of the tube has to be as least as high as the height of the largest stack of containers, so that that the highest stack of containers can be extracted in a single operation. Accordingly, when used in an enclosed space such as a warehouse, the maximum height of the stacks is restricted by the need to accommodate the tube of the load handler. Furthermore, the system is not well adapted for the selection of a single container from a multi-product stack.
Online retail businesses selling multiple product lines, such as online grocers and supermarkets, require systems that are able to store tens or even hundreds of thousands of different product lines. The use of single-product stacks in such cases can be impractical, since a very large floor area would be required to accommodate all of the stacks required. Furthermore, it can be desirable only to store small quantities of some items, such as perishables or infrequently-ordered goods, making single-product stacks an inefficient solution.
Accordingly, for some applications, the use of multi-product stacks, in which the containers making up each stack may hold different products, is favoured in order to maximise the storage density of the system. The stored items must remain accessible reasonably quickly and easily, so that a plurality of different items required to fulfil a customer order can be picked from the storage system in an efficient way, even if some of the items required are stored in a lower level of a stack, underneath several other containers.
International patent application WO 98/049075A (Autostore), the contents of which are incorporated herein by reference, describes a system in which multi-product stacks of containers are arranged within a frame structure. A system of this type is illustrated schematically in FIGS. 1 to 5 of the accompanying drawings.
As shown in FIGS. 1 and 2, stackable containers, known as bins 10, are stacked on top of one another to form stacks 12. The stacks 12 are arranged in a grid frame structure 14 in a warehousing or manufacturing environment. FIG. 1 is a schematic perspective view of the frame structure 14, and FIG. 2 is a top-down view showing a stack 12 of bins 10 arranged within the frame structure 14. Each bin 10 typically holds a plurality of product items (not shown), and the product items within a bin 10 may be identical, or may be of different product types depending on the application.
The frame structure 14 comprises a plurality of upright members 16 that support horizontal members 18, 20. A first set of parallel horizontal members 18 is arranged perpendicularly to a second set of parallel horizontal members 20 to form a plurality of horizontal grid structures supported by the upright members 16. The members 16, 18, 20 are typically manufactured from metal. The bins 10 are stacked between the members 16, 18, 20 of the frame structure 14, so that the frame structure 14 guards against horizontal movement of the stacks 12 of bins 10, and guides vertical movement of the bins 10.
The top level of the frame structure 14 includes rails 22 arranged in a grid pattern across the top of the stacks 12. Referring additionally to FIGS. 3 and 4, the rails 22 support a plurality of robotic load handling devices 30. A first set 22a of parallel rails 22 guide movement of the load handling devices 30 in a first direction (X) across the top of the frame structure 14, and a second set 22b of parallel rails 22, arranged perpendicular to the first set 22a, guide movement of the load handling devices 30 in a second direction (Y), perpendicular to the first direction. In this way, the rails 22 allow movement of the load handling devices 30 laterally in two dimensions in the horizontal X-Y plane, so that a load handling device 30 can be moved into position above any of the stacks 12.
The load handling devices 30 are further described in Norwegian patent number 317366, the contents of which are incorporated herein by reference. FIGS. 3(a) and 3(b) are schematic perspective views of a load handling device 30 from the rear and front, respectively, and FIG. 3(c) is a schematic front perspective view of a load handling device 30 lifting a bin 10.
Each load handling device 30 comprises a vehicle 32 which is arranged to travel in the X and Y directions on the rails 22 of the frame structure 14, above the stacks 12. A first set of wheels 34, consisting of a pair of wheels 34 on the front of the vehicle 32 and a pair of wheels 34 on the back of the vehicle 32, is arranged to engage with two adjacent rails of the first set 22a of rails 22. Similarly, a second set of wheels 36, consisting of a pair of wheels 36 on each side of the vehicle 32, is arranged to engage with two adjacent rails of the second set 22b of rails 22. Each set of wheels 34, 36 can be lifted and lowered, so that either the first set of wheels 34 or the second set of wheels 36 is engaged with the respective set of rails 22a, 22b at any one time.
When the first set of wheels 34 is engaged with the first set of rails 22a and the second set of wheels 36 is lifted clear from the rails 22, the wheels 34 can be driven, by way of a drive mechanism (not shown) housed in the vehicle 32, to move the load handling device 30 in the X direction. To move the load handling device 30 in the Y direction, the first set of wheels 34 is lifted clear of the rails 22, and the second set of wheels 36 is lowered into engagement with the second set of rails 22a. The drive mechanism can then be used to drive the second set of wheels 36 to achieve movement in the Y direction.
The load handling device 30 is equipped with a crane device 40. The crane device 40 comprises a cantilever arm 42 that extends laterally from the top of the vehicle 32. A gripper plate 44 is suspended from the cantilever arm 42 by four cables 46. The cables 46 are connected to a winding mechanism (not shown) housed within the vehicle 32. The cables 46 can be spooled in or out from the cantilever arm 42, so that the position of the gripper plate 44 with respect to the vehicle 32 can be adjusted in the Z direction.
The gripper plate 44 is adapted to engage with the top of a bin 10. For example, the gripper plate 44 may include pins (not shown) that mate with corresponding holes (not shown) in the rim that forms the top surface of the bin 10, and sliding clips (not shown) that are engageable with the rim to grip the bin 10. The clips are driven to engage with the bin 10 by a suitable drive mechanism housed within the gripper plate 44, which is powered and controlled by signals carried through the cables 46 themselves or through a separate control cable (not shown).
To remove a bin 10 from the top of a stack 12, the load handling device 30 is moved as necessary in the X and Y directions so that the gripper plate 44 is positioned above the stack 12. The gripper plate 44 is then lowered vertically in the Z direction to engage with the bin 10 on the top of the stack 12, as shown in FIG. 3(c). The gripper plate 44 grips the bin 10, and is then pulled upwards on the cables 46, with the bin 10 attached. At the top of its vertical travel, the bin 10 is accommodated beneath the cantilever arm 42 and is held above the level of the rails 22. In this way, the load handling device 30 can be moved to a different position in the X-Y plane, carrying the bin 10 along with it, to transport the bin 10 to another location. The cables 46 are long enough to allow the load handling device 30 to retrieve and place bins from any level of a stack 12, including the floor level. The vehicle 32 is sufficiently heavy to counterbalance the weight of the bin 10 and to remain stable during the lifting process. The weight of the vehicle 32 may be comprised in part of batteries that are used to power the drive mechanism for the wheels 34, 36.
As shown in FIG. 4, a plurality of identical load handling devices 30 are provided, so that each load handling device 30 can operate simultaneously to increase the throughput of the system. The system illustrated in FIG. 4 includes two specific locations, known as ports 24, at which bins 10 can be transferred into or out of the system. An additional conveyor system (not shown) is associated with each port 24, so that bins 10 transported to a port 24 by a load handling device 30 can be transferred to another location by the conveyor system, for example to a picking station (not shown). Similarly, bins 10 can be moved by the conveyor system to a port 24 from an external location, for example to a bin-filling station (not shown), and transported to a stack 12 by the load handling devices 30 to replenish the stock in the system.
Each load handling device 30 can lift and move one bin 10 at a time. If it is necessary to retrieve a bin 10 (“target bin”) that is not located on the top of a stack 12, then the overlying bins 10 (“non-target bins”) must first be moved to allow access to the target bin 10. This is achieved in an operation referred to hereafter as “digging”.
Referring to FIG. 5, during a digging operation, one of the load handling devices 30 sequentially lifts each non-target bin 10a from the stack 12 containing the target bin 10b and places it in a temporary location on top of another stack 12. The target bin 10b can then be accessed by the load handling device 30 and moved to a port 24 for further transportation. In FIG. 5, eight non-target bins 10a have been placed into temporary positions, and the target bin 10b has been withdrawn from the stack 12 by the load handling device 30. Once the target bin 10b has been extracted, the non-target bins 10a can be replaced one-by-one in the stack 12 by the load handling device 30, to restore the original order of the stack 12, less the target bin 10b. 
Each of the load handling devices 30 is under the control of a central computer. Each individual bin 10 in the system is tracked, so that the appropriate bins 10 can be retrieved, transported and replaced as necessary. For example, during a digging operation, the temporary locations of each of the non-target bins 10a is logged, so that the non-target bins 10a can be replaced in the stack in the correct order.
The system described with reference to FIGS. 1 to 5 has many advantages and is suitable for a wide range of storage and retrieval operations. In particular, it allows very dense storage of product, and it provides a very economical way of storing a huge range of different items in the bins 10, while allowing reasonably economical access to all of the bins 10 when required for picking.
However, there are some drawbacks with such a system, which all result from the above-described digging operation that must be performed when a target bin 10b is not at the top of a stack 12.
In a typical installation, the stacks 12 can be up to twenty-four bins tall. This means that, to access a target bin 10b that is towards the bottom of the stack 12, it is necessary first to remove a large number of non-target bins 10a. As shown in FIG. 5, the non-target bins 10a stored in temporary locations restrict access to the rails 22, making a relatively large area of the store inaccessible to other load handling devices 30 during a digging operation. Furthermore, the digging process consumes a large part of the operating time of the load handling devices 30, reducing the efficiency and increasing the cost of the system, because it is necessary to provide multiple load handling devices 30 to avoid delays.
The digging operation is relatively slow, and it can take many minutes to access a target bin 10b that is situated towards the bottom of a stack 12. The time taken is increased when the target stack 12 is already being used for temporary storage of a non-target bin 10a as part of the digging operation for another target bin 10b. Thus multiple digging operations have a cumulative negative effect on the efficiency of the system. To mitigate delays caused by digging operations, the picking process needs to be orchestrated a long time in advance, typically at least thirty minutes. As a result, the system cannot react quickly to changes in demand without significant loss of productivity.
Another strategy for minimising the time spent on digging operations is to arrange the bins 10 in the stack 12 so that the most frequently accessed bins 10 are close to the top of the stacks 12. However, this approach becomes limited in applications in which products are picked to assemble orders that consist of a relatively large number of products selected from a very large number of product lines, because it is always likely that a small number of comparatively rarely-ordered products, requiring a time-consuming digging operation, will be present in each order.
Against this background, it would be desirable to provide systems and methods that reduce or mitigate some of the above-described problems.